The　oligomerization　of　olefins　is　important　to　the　production　of　the　high　hydrocarbons.　In　the　oligomerization　process,　it　is　always　accompanied　by　the　reaction　of　hydrogenation.　In　the　present　article,　we　report　that　surface　carbon　affects　the　ethylene　(C2H4)　hydrogenation　on　the　supported　cobalt　catalysts.　Co/SiO2　catalyst　is　carburized　with　CO　and　Co3C　structure　as　well　as　Co　structure　is　revealed　with　Transmission　Electron　Microscopy　(TEM).　We　investigate　ethylene　hydrogenation　on　the　Co/SiO2　catalyst　and　the　carburized　Co3C-Co/SiO2　catalyst.　It　exhibits　a　high　selectivity　of　C-4　products　on　the　carburized　Co3C-Co/SiO2　catalyst.　We　further　apply　Density　Functional　Theory　(DFT)　calculation　to　study　the　effect　of　surface　carbon　on　the　dimerization　of　ethylene.　Co(0001)　and　Co3C(221)　are　chosen　to　represent　the　Co/SiO2　and　Co3C-Co/SiO2　catalysts,　respectively.　The　dimerization　of　ethylene　occurs　by　CCH3*　species　coupling　instead　of　CH2CH2*　coupling.　The　effective　barrier　of　CCH3*　coupling　on　Co(0001)　is　higher　than　that　on　Co3C(221).　This　explains　the　high　selectivity　to　C-4　products　on　the　carburized　Co3C-Co/SiO2　catalyst.　Our　study　provides　experimental　and　theoretical　insight　for　the　effect　of　surface　carbon　to　the　reaction　of　ethylene　with　hydrogen.　It　may　guide　us　to　reasonable　design　of　the　catalyst　for　olefin　oligomerization.